A spin polarized current refers to a current which is applied with a degree of freedom of spin to the current caused by the charge, and is a current keeping a degree of freedom of charge and a degree of freedom of spin simultaneously. The magneto-resistance effect generated by allow the spin polarized current to flow between the ferromagnetic electrodes through a nonmagnetic layer is great and thus is referred to as giant magneto-resistance effect (GMR). Such an effect is applied to products such as a magnetic head and a sensor. In addition, it is well known that the effect generated in a configuration in which a tunnel film is used instead of a nonmagnetic layer is referred to as tunneling magneto-resistance effect (TMR), and a characteristic better than GMR can be obtained. These elements are passive elements in which outputs are generated based on a relative spinning angle between the ferromagnetic electrodes. Herein, the passive elements are used as the magnetoresistive elements. When the nonmagnetic layer is used as a semiconductor, there is an amplifying function in the semiconductor in addition to the magneto-resistance effect. Thus, active elements in spintronics have attracted attention. In Patent Documents 1 and 2, Spin-MOSFET is proposed in which magneto-resistance effect generated by spin polarized current flowing through a semiconductor is used.
It is known that there are two main factors for obtaining a giant magneto-resistance ratio in a structure where a nonmagnetic layer is sandwiched by two ferromagnetic layers. One is to increase the magneto-resistance caused by the spin of the ferromagnetic layers, thus increase the resistance. The other is to decrease the self-resistance of the element generating the magneto-resistance itself, in addition, the magneto-resistance ratio (MRR) is represented by MRR=(RAP−RP)/RP×100, wherein, RAP is the resistance when the magnetization orientations of the 2 ferromagnetic layers are anti-parallel, and RP is the resistance when the magnetization orientations of the 2 ferromagnetic layers are parallel. Further, the magneto-resistance ΔR caused by spin is represented by the difference of RAP and RP, i.e., ΔR=RAP−RP. That is, it is known that a large magneto-resistance ratio can be obtained when RP is small and RAP is large.
Usually, the nonmagnetic layer plays an important role in the resistance of the element in the magnetoresistive elements. Generally, when GMR is used, a metal material is used in the nonmagnetic layer, thus the resistance of the element is small. In other words, the RP becomes small. Conversely, when TMR is used a tunnel insulating material is used in the nonmagnetic layer, thus the resistance of the element is large. That is, the RP is increased. However, it is known that a larger ΔR can be obtained in the case of TMR than in the case of GMR. As a result, it is known that a larger magneto-resistance ratio can be obtained in the case of TMR than in the case of GMR.
It will be more complex when the nonmagnetic layer is made of semiconductor material than in the case mentioned above. The reason is considered to be that the semiconductor material is a material which can transport spins easily and a larger ΔR can be obtained compared to the case when the nonmagnetic layer is made of metals. Additionally, when the nonmagnetic layer is made of a semiconductor material, the RP may also be smaller and a higher magneto-resistance ratio can be obtained compared to that when the nonmagnetic layer is made of a tunnel insulating, material. Also, when the nonmagnetic layer is made of a semiconductor material, the element may also be functioned as an active element.
As a method to decrease the resistance, it is considered to decrease the resistance of each layer. However, generally speaking, if the material is changed, the scattering caused by spin will change and the magneto-resistance ratio is not necessarily increased. In addition, when the path of the current flowing is in a sequence of the ferromagnetic layer, the tunnel layer, the semiconductor layer, the tunnel layer, and the ferromagnetic layer, a method of measuring the voltage between the interfaces of the semiconductor layer and the ferromagnetic layer is considered. However, according to the Non-Patent Document 2, the resistance in the entire circuit can be decreased to about a half by the method mentioned above, however, the output caused by spin will also decrease to a half, thus a problem is caused that the magneto-resistance ratio cannot be increased.